Technical Field
The present invention relates to coded modulation, and more particularly to quasi-cyclic (QC) Low Density Parity Check (LDPC) coded modulation with serial turbo decoding for data transport in optical communications systems.
Description of the Related Art
Digital coherent technology has made 100G over transoceanic distance using quadrature phase-shifted-keying (QPSK) become commercially available to improve spectral efficiency. However, when further increasing the spectral efficiency beyond QPSK, the distance is much shorter due to the high required signal-to-noise ratio (SNR). The conventional state-of-the-art forward error correction (FEC) is using capacity-approaching low density parity check (LDPC) codes at 20%-30% OH content, which improves the FEC limit Q to about ˜5 dB Q with less than 12 dB net coding gain (NCG). Even though conventional systems and methods have used 8 quadrature amplitude modulation (8QAM) for transpacific cables, the system margin is still barely above the FEC limit, and thus is unreliable, and risky to use due to the higher fiber loss from aging and cable cut/repair.
Different coding methods have been proposed to increase the NCG, and to lower the error floor. For example, conventional systems and methods include a concatenated system considering two block codes as an outer code. These codes include a 16 times interleaved byte-oriented (255,239) Reed Solomon (RS) code and a code consisting of two interleaved extended three-error correcting Bose Chaudhuri Hocquenghem (BCH) (1020,988) codes that achieves an NCG of 8.42 dB at a bit-error rate of 10−13 and the NCG of 9.7 dB when the concatenated code based on the two interleaved BCH codes is used as the outer code. Some conventional methods propose the concatenation of a spatially-coupled type irregular LDPC code with a BCH code with an NCG of 12.0 dB at a BER of 10−15, with 25.5% redundancy. They proposed a spatially-coupled-like LDPC code to achieve a better error-floor performance than common spatially-coupled LDPC code.